Aluminium sulfate

Aluminium sulfate (anhydrous) IUPAC name Aluminium sulfate Other names Cake alum Filter alum Papermaker's alum Sulfuric acid, aluminum salt (3:2) Identifiers InChI InChI=1/2Al.3H2O4S/c;;3*1-5(2,3)4/h;;3*(H2,1,2,3,4)/q2*+3;;;/p-6 InChIKey DIZPMCHEQGEION-CYFPFDDLAS Standard InChI InChI=1S/2Al.3H2O4S/c;;3*1-5(2,3)4/h;;3*(H2,1,2,3,4)/q2*+3;;;/p-6 Standard InChIKey DIZPMCHEQGEION-UHFFFAOYSA-H CAS number [10043-01-3] EC number 233-135-0 RTECS BD1700000 ChemSpider 23233 PubChem 24850 Properties Chemical formula Al2(SO4)3 Molar mass 342.15 g/mol Appearance white crystalline solid, hygroscopic Density 2.71 g/cm3 Melting point 770 °C decomp. Solubility in water 31.3 g/100 ml (0 °C) 36.4 g/100 ml (20 °C) 98.1 g/100 ml (100 °C) Solubility slightly soluble in alcohol, dilute mineral acids Refractive index (nD) 1.47 Hazards Material safety data sheet (MSDS) ICSC 1191 EU index number not listed GHS pictograms GHS signal word DANGER GHS hazard statements H290, H302, H315, H318, H411 GHS precautionary statements P234, P264, P270, P273, P280, P301+312, P302+352, P305+351+338, P310, P321, P330, P332+313, P337+313, P362 Flash point non-flammable Related compounds Other cations Gallium sulfate Indium sulfate Thallium sulfate Other compounds See Alum Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Aluminium sulfate octadecahydrate IUPAC name Aluminium sulfate — water (1:18) Other names Alunogenite Identifiers InChI InChI=1/2Al.3H2O4S.18H2O/c;;3* 1-5(2,3)4;;;;;;;;;;;;;;;;;;/h; ;3*(H2,1,2,3,4);18*1H2/q2*+3;; ;;;;;;;;;;;;;;;;;;;/p-6 InChIKey AMVQGJHFDJVOOB-CYFPFDDLAL Standard InChI InChI=1S/2Al.3H2O4S.18H2O/c;;3 *1-5(2,3)4;;;;;;;;;;;;;;;;;;/h ;;3*(H2,1,2,3,4);18*1H2/q2*+3; ;;;;;;;;;;;;;;;;;;;;/p-6 Standard InChIKey AMVQGJHFDJVOOB-UHFFFAOYSA-H CAS number [7784-31-8] EC number 233-135-0 RTECS WS5697000 ChemSpider 15847672 Properties Chemical formula Al2(SO4)3·18H2O Molar mass 666.42 g/mol Appearance white crystalline solid Density 1.69 g/cm3 Melting point 86.5 °C Refractive index (nD) 1.47 Structure Crystal structure monoclinic Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Aluminium sulfate hexadecahydrate IUPAC name Aluminium sulfate — water (1:16) Identifiers CAS number [16828-11-8] EC number 233-135-0 RTECS WS5696000 Properties Chemical formula Al2(SO4)3·16H2O Molar mass 630.40 g/mol Appearance white crystalline solid Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Aluminium sulfate, Al₂(SO₄)₃, is an industrial chemical used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper manufacturing. It is a food additive, specifically a firming agent, referred to as E520.

Aluminium sulfate is sometimes incorrectly referred to as alum but alums are closely related compounds typified by KAl(SO₄)₂^.12H₂O. The anhydrous form occurs naturally as a rare mineral millosevichite, found e.g. in volcanic environments and on burning coal-mining waste dumps. Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate Al₂(SO₄)₃·16H₂O and octadecahydrate Al₂(SO₄)₃·18H₂O are the most common. The heptadecahydrate, whose formula can be written as [Al(H₂O)₆]₂(SO₄)₃·5H₂O, occurs naturally as the mineral alunogenite.

Preparation

Aluminium sulfate may be made by aluminium hydroxide, Al(OH)₃, in sulfuric acid, H₂SO₄:

2 Al(OH)₃ + 3 H₂SO₄ → Al₂(SO₄)₃·6H₂O

Uses

Aluminium Sulfate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes impurities to coagulate which are removed as the particulate settles to the bottom of the container or more easily filtered. This process is called coagulation or flocculation.

When dissolved in a large amount of neutral or slightly-alkaline water, aluminium sulfate produces a gelatinous precipitate of aluminium hydroxide, Al(OH)₃. In dyeing and printing cloth, the gelatinous precipitate helps the dye adhere to the clothing fibers by rendering the pigment insoluble.

Aluminium sulfate is sometimes used to reduce the pH of garden soil, as it hydrolyzes to form the aluminium hydroxide precipitate and a dilute sulfuric acid solution. An example of what changing the pH level of soil can do to plants is visible when looking at the Hydrangea macrophylla. The gardener can add Aluminum sulfate to the soil to reduce the pH level which in turn will result in the flowers of the Hydrangea turning a different color.

Aluminium sulfate is the active ingredient of some antiperspirants; however, beginning in 2005 the US Food and Drug Administration no longer recognized it as a wetness reducer.

Aluminium sulfate is usually found in baking powder, where there is controversy over its use due to concern regarding the safety of adding aluminum to the diet.

In construction industry it is used as waterproofing agent and accelerator in concrete. Another use is a foaming agent in fire fighting foam.

It is also used in styptic pencils, and pain relief from stings and bites; it is the active ingredient in popular pain relief products such as Stingose.

It can also be very effective as a molluscicide, killing spanish slugs.

Chemical Reactions

The compound decomposes to γ−alumina and sulfite when heated between 580 and 900°C. It combines with water forming hydrated salts of various compositions.

Produces calcium aluminate with the evolution of sulfate when calcined with calcium sulfate:

Al₂(SO₄)₃ + CaSO₄ → Ca(AlO₂)₂ + 4 SO₃

Aluminium sulfate reacts with sodium bicarbonate to which foam stabilizer has been added, typically made from saponin, licorice root, or hydrolyzed animal protein in aqueous solution, forming fire-extinguishing foams, producing carbon dioxide

Al₂(SO₄)₃ + 6 NaHCO₃ → 2 Al(OH)₃ + 3 Na₂SO₄ + 6 CO₂

The carbon dioxide is trapped by the foam stabilizer and creates a thick foam which will float on top of hydrocarbon fuels and seal off access to atmospheric oxygen, smothering the fire. Chemical foam was unsuitable for use on polar solvents such as alcohol, as the fuel would mix with and break down the foam blanket. The carbon dioxide generated also served to propel the foam out of the container, be it a portable fire extinguisher or fixed installation using hoselines. Chemical foam is considered obsolete in the United States and has been replaced by synthetic mechanical foams, such as AFFF which have a longer shelf life, are more effective, and more versatile, although some counties such as Japan and India continue to use it.

References

Notations

• Pauling, Linus (1970). General Chemistry. W.H. Freeman: San Francisco. ISBN 0-486-65622-5. 

External links

• International Chemical Safety Card 1191
• NLM Hazardous Substances Data Bank entry for Aluminum sulfate
• NIOSH Pocket Guide to Chemical Hazards 0024
• IPCS Environmental Health Criteria 194: Aluminium
• FAO/WHO Joint Expert Committee on Food Additives (JECFA) monograph: Aluminium (WHO Food Additives Series 24)
• Aluminum and health